I believe the MD600 runs a flavour of Linux, so it is possible for a virus to exist there, certainly when the Microsoft bit locker virus was prevalent it was possible for NAS drives to become affected as well as the host PC.
VK may well be able to provide more in depth detail?
I thought the MD600 runs Windows XP embedded, but could be wrong. If it does, then most of the unneeded Windows services are likely to be disabled, which should harden it against viruses, however it is probably not immune.
The TwinStore and MD600 are stripped down Linux boxes, with the mostly minimum amount of executables necessary to run the Core(MD600) and the Store (TwinStore & MD600). (The Control series are Windows XP embedded).
They are also file servers (mostly designed to serve FLAC files to Sooloos). They could possibly hold files infected with viruses but probably aren't a large enough target (numerically) to warrant any specific attacks against them.
Every Sooloos FLAC file seems to have a md5 checksum associated with it, potentially to allow for a re-check of integrity, but I don't know how to initiate something like that.