I've also been having problems with my Treo 750v and the miniSD card (2GB SanDisk Ultra II).
When I insert the card, the Treo sees it and can run programs that are installed on it. When I return the Treo after it has been off for a while the card is no longer recognised and the memory manager claims that no storage card is installed. At this point all I can do is either take the card out and reinsert it, or otherwise to a soft reset. The time it takes for the Treo to loose its connection to the card varies; sometimes I can turn it on after a few minutes and the card is gone, sometimes it still works after a couple of hours.
When I came across this discussion yesterday, I phoned up Vodafone and reported my issue. According to the customer service rep, this is
not on their list of known issues but he agreed to an exchange anyway. I'll get the new device today. I hope that'll fix it, but by now have my doubts.
Someone else suggested formatting the SD card, citing issues with FAT16 vs FAT32. I might try that if the new Treo still suffers the same issue.
In another thread on this topic, someone reported that such an issue was observed on some other PPC device and was fixed by a ROM upgrade. I emailed Palm about it. Palm eventually got back to me today saying:
This may occur only with cards that are not Palm branded. If possible please try to use a Palm card to test it.
Either that's a lame excuse, or they're trying to force people into buying their overpriced own branded accessories by making their hardware incompatible with the industry standard. Unless SanDisk don't know how to make miniSD cards...
I guess my next step - after the reformatting - will be to throw £60 at a Palm branded storage card; I don't expect that would work any better than my current card, but at least then Palm can't argue that I'm using "unsupported hardware".