I had the memory problems to, but the cause is that when you simply unplug the N900 from your pc (without Eject or save hardware removal) the phone can sometimes think it is still connected. (only had this problem connected to Windows machines) Linux gives no issues. But simply turning your phone off and on again will solve this.

NOTE: if you haven't found this out allready the N900 locks the access to your N900 HDD and Flash Card when connected to a pc as mass storage mode. Disconnecting the nokia without the Eject or Save Removal option (Windows) can cause the Memory access. This wil only be solved after a reboot of your phone.

P.S. the N900 has roughly 900MB storage to install apps, i doubt that anyone can get that full within 2 weeks.