I have been doing some experimenting with the DIR-505 and its ability to "serve" a USB storage device to clients on its network. I have found the following:
USB flash drives are not a problem, as long as they are formatted for FAT32 (most are)
The DIR-505 does not supply enough power from its USB port to power most USB hard drives, and especially CD/DVD/BD drives. However, if your drive can accept external power, either through a dedicated jack or by use of a USB Y cable (for example,
this one from Amazon), then it will work if the drive uses the MBR partition table type and has a FAT32 partition. My experiments show that a drive with a GPT table type (common on Macs and also supported by Windows 7) will not be recognized by the DIR-505. I have not yet done the experiment to see if the DIR-505 will recognize an NTFS partition on an MBR drive.