You are right - I was using MTP mode, no drive letter is assigned. I also got that error with Windows Media Player, but next time I simply chose to open 'Archos 5S' in Windows Explorer; inside it displayed two separate entities, 'Internal Hard Drive' and 'SD Card'. Then, I was able to manually copy media files from my PC to either 'Internal Hard Drive' or 'SD Card' on A5A. I found it more convenient than to "sync" with those overcomplicated tools.
Here's a related topic on ArchosFans forum