After a frustrating couple of weeks battling to reliably get music onto my N91 and have finally found a way to transfer files without the program crashing/stalling halfway though the transfer. I have certain MP3's which will reliably crash PC-Suide, Media Player (dont have a mac, yeh I know, I bet it works just fine on OS-X, works great on linux too but you have to select 'yes' for each and every file you transfer), and even Mass Storage mode (which oddly doesnt work on SuSE 10.0, and other mass storage devices work fine ... gotta love standards eh?)
Anyway ... the solution (for Windows XP machines) is to use 'obex commander' ... a free tool (donations) that uses the OBEX ftp protocol to transfer files to your phone, while this is primarily a bluetooth/IR thing, it is also available on the USB cable.
OBEX commander also lets you control exactly how you connect to the phone, I reccomend using the 'PC Suite' mode and connecting as either PCCS or Object Push (With object push, click refresh to retrieve the directory listing after you connect).
There does appear to be a bug in the app, in that it allows you to click on folders to transfer them, and it claims to be creating folders etc, however the folders never get created and all the files attempt to transfer, but fail as they try to be written. However I can live with that give that I can select as many tracks as I like, click 'send' and know that they will transfer 99.99% of the time
If the transfer does fail for some reason, then you can just click connect and try again ... unlike my experiences with all the other ways, which generally involved closing down the app, disconnecting the cable and starting the app up etc...
Anyway ... it works for me so I'm chuffed 😃 ... you can dowload it here : intradarma.com