Migrating from HDD to SSD is a practical way of enabling legacy computer-based systems in use in the military and aerospace sectors to provide several more years of reliable service. Brian McSloy explains disk formatting and cloning.
Many computer-based systems in use in the military and aerospace sectors were designed more than 20 years ago. These systems include mission computers and dataloggers installed on military aircraft, flight management systems on commercial aircraft, ground-based maintenance and diagnostic systems, and simulation and training systems.
When new these systems were fitted with then state-of-the-art hard disk drives (HDDs) and a popular way of connecting the drives to their host computers was the small computer system interface (SCSI, “scuzzy”), which was standardised in 1986 as the SCSI parallel interface (SPI) 8-bit wide, single-ended bus. The standard evolved through a number of iterations, doubling the number of data lines and incorporating differential signalling (both allowing the transfer rate to significantly increase) before finally being superseded by the serial attached SCSI (SAS) interface.
With their moving parts, the electromechanical SCSI drives within these legacy systems are at increased risk of failing and will certainly not be as responsive as they once were – see panel ‘HDD Failure Modes’. This presents several problems.
Firstly, the original HDDs are long obsolete, and the OEM may no longer be around. Secondly, it would need to be an exact model/variant replacement as SCSI was something of a loose standard, and the OEMs of host systems and the OEMs of the SCSI drives sometimes made tweaks in order to assure exclusive compatibility between their products: think secret handshake.
Thirdly, reconditioned [exact model] units, if they can be found, have little if any warranty and their life expectancy is unknown. Note: in many cases those who had the foresight to buy spares back in the day are finding to their horror that they do not work. For example, many HDDs have heads that rest on the disk surface when not in use. Over time (years in storage), the heads can stick. When the drive is powered, the disk surface is scratched, and sometimes the head is ripped from its suspension mechanism.
Lastly, if the host computer is used in safety- or mission-critical applications it will have been certified and cannot be modified without recertification: a lengthy and expensive process, as is making the modification.
The failure of the HDDs within systems that are more than 20 years old is a case of when, not if. Accordingly, to ensure the reliability of the host system and extend its life, the only logical recourse is to replace the electromechanical SCSI drive with a solid-state-based clone.
Cloning
Essentially, an HDD is a set of spinning platters coated with a ferromagnetic material, a read/write head mounted on an actuator arm, and electronics for interpreting commands from a host system’s software via its operating system (OS). In order to be treated as a storage device by an OS, the HDD must be formatted. This occurs at two levels.
The first, physical formatting (a.k.a. low-level formatting – LLF), defines the physical structure of the disk in terms of tracks, sectors and headers. Part of this is done at the point of manufacture and cannot be changed and part is achieved via a SCSI command, normally at first set up.
The second level is user-level formatting (a.k.a. high-level formatting – HLF), which creates a file system (such as NTFS or FAT32) and builds boot sectors and a file allocation table. User-level formatting is also performed at the point of manufacture, but, as the name indicates, is also something the end user can do.
Accordingly, for the SSD to be treated by the host computer as if it were the original HDD, it must be cloned – where cloning makes a block-for-block exact replica of the HDD including the OS, boot sector, system files, the file system structure, partition table, recovery partitions and data. By contrast, copying duplicates only the data (folders and files) from one drive to another.
Cloning a modern HDD (i.e. one that uses SATA or NVMe, for example) is relatively easy. The equipment needed comprises a computer that can access both the source drive and the target SSD, plus cables and/or docking stations. Adaptors, such as SATA to USB, may also be required. As for the software needed to control the cloning operation, there are many commercial offerings available.
Cloning an old SCSI drive is far more complex because it uses a different connector type and protocols: i.e. you cannot simply plug a SCSI drive into a modern PC without special hardware.
Specialised solid-state SCSI drives are commercially available, as are the software and hardware needed to clone the SCSI HDD. For instance, Solid State Disks Limited’s (SSDL’s) SCSIFlash technology combines proven SCSI drive architectures (SASI, SCSI-1, SCSI-2) with industry-standard, solid-state CompactFlash (CF) card technology, and has been used to replace SCSI HDDs made by Seagate, HP, Sony and many other OEMs.
A clone of the source SCSI drive’s disk needs to reside on the CF. To make the clone, SSDL can supply DuplicatorPlus (see figure 1). It can be used out of the box and without needing a PC to clone the image of most SCSI-based drives. It can connect directly to a source drive and cloning starts at the press of a button.
Accordingly, solid-state, swap-in replacements can take the place of SCSI drives in use (and at risk of failure) in many legacy computer-based military and aerospace systems. And in most cases, maintenance engineers can easily make the switch themselves. Note, if a secret handshake was agreed decades ago between the OEMs of the drive and the host, all is not lost. Specialised analysis equipment can be used to capture signal timings and data exchanges so that they might be programmed into the SCSIFlash firmware.
In summary, cloning is a common practice and is very easy to do if the source drive is relatively modern. Older drives that use the SCSI standard (or a tweaked version) present certain challenges, particularly if the host computer cannot be modified, but solutions do exist.
