As stated, for convenience, you should consider a large NAS server. It can be setup in minutes and requires little maintenance. If you like, you can put new drives into the NAS, copy all of your files onto it, then store the 15 drives on the shelf. So, these "eggs" will not be in the same basket. Of course if these files are critical, consider adding another backup location in addition the the NAS and Externals.
You cannot attach a wireless adapter to the external drive, because the USB port is only for data transfer and does not have an OS on the drive (like a NAS would have) to access the WiFi adapter. You can plug an external drive into the USB port of a Router for a single drive.
I'm thinking of getting a USB wifi adapter to plug into a desktop PC hard drive (manufactured by Dell) to access my xfinity wifi hot wire. Is it possible to plug such a device into a hard drive? If...
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Any of your drives that are connected internally or externally continuously to a computer can be shared and should be available on your network at any time that computer is up.