I bought a NOOK Color when it was new on the market, and use it to read both books and magazines. Lately, though, I'm more likely to read magazines on a 10" Android tablet using the NOOK reader application for Android. It works well enough, with one glaring exception: periodically it refuses to download new issues of magazines, claiming the disk is full (which it is not).
I'm not sure exactly what causes NOOK to think it has a storage limit, but given the existence of such a limit I can see why NOOK believes it is reaching said limit. When you are done with content on an NOOK device, you have the options to either delete or archive the content. According to the FAQ for the NOOK app, archiving content theoretically removes it from your device but keeps it available in the cloud, from whence you can download it again if you want another look. Deleting content deletes it permanently; it does not remain available from the cloud. So archiving is generally the preferred route ... except that the part about deleting it is not entirely true.
After once again running out of space, I decided to go spelunking with a file manager. I found a folder (/Nook/Content/.Temp/Drp/) containing .epub folders for various archived issues. All told, a few hundred megabytes of storage were being consumed by issues that theoretically had been deleted from my tablet.
I did a web chat with a customer support person from Barnes and Noble, and at one point (s)he had me sign out of the NOOK application and then sign back in (which syncs the library). I pretty much never sign out of the app, having no particular reason to do so. Woo-hoo, all the archived issues were gone! Sadly, so was an issue I had downloaded but not yet read. There was no problem downloading it again, other than consumption of time and bandwidth.
I asked the support person to file a couple of bug reports. As far as I'm concerned, having to sign out and back in to free up space is dopey. I might add that I had previously synced the library (without signing out), and that did not deleted the .epub folders for archived content (despite the NOOK app correctly listing the content has having been archived). I also think that losing active (not deleted, not archived) content when you sign out is a (separate) bug.
We'll see if those things get fixed, but in the meantime my workaround is to use the file manager to delete .epub folders with old dates whenever the NOOK app tells me I'm out of space.