.ros appers to be used for Marvell-powered switches. So far it has been spotted in some Dell and Netgear switches

Header Edit

The offset and length is sometimes bigendian, sometimes little-endian, probably depending on CPU-type of the device.

Offset Comment
00-0Fh Devicename?
10-1Fh Type of compression?
20-2Fh ???
Filemetadata, in 0x20 blocks
30-3Fh ASCII filename
40-43h Offset in file, excluding first 0x20 bytes

Length of file

47-4Fh ???

Compression Edit

Some of the files are uncompressed, some are compressed with LZMA (recognizable by 5D 00 00 80 header), some are compressed with LHA, some are ZIP-ish, etc. Interestingly, all parts are mostly decompressable by standard tools (lharc, zip, lzma, etc.)

I have not yet been able to replace a LHA-part yet, succesfully. Probably there is a minor difference between the format the bootloader expects and what 'LHa for UNIX V 1.14i' generates. If I don't use any parameters it complains about the compression method (lh7) used. If I use compression method lh5, I get the following error on my PowerConnect 3324:

Autoboot in 2 seconds - press RETURN or Esc. to abort and enter prom.
Preparing to decompress...

Decompressing SW from image-1
There's an extended compress header.
There's an extended compress header.