Nothing really stands out as causing the problem you are having, unfortunately. Minor side issues are: 1) the PerfMon plugin was deprecated long ago and should be changed to UsageMonitor, and 2) it is not a good idea to use variables with names containing colons or other symbols. Rainmeter's parser can get confused since colons have a special meaning.
Since you have been messing with the drives, your Performance Counter database may have become corrupted. It is easy for this to happen, even Windows Updates corrupts it sometimes. Go to the manual here, and scroll to the bottom of the page. Follow the instructions to reset your database. You'll need to exit Rainmeter and restart it, or possibly reboot your machine, once you are done.
If the problem persists, your next move would be to confirm that what you are seeing is actually coming from the data source. From your Windows Tools Control Panel, run the Performance Monitor app. Click the "Performance Monitor" branch in the left pane and click the green "+" icon. Scroll down to the "Logical Disk" item and expand it. Select the "Disk Read Bytes/sec" counter and in the instance box, select your problematic P: and M: drives and click "Add >>". Repeat this with the "Disk Write Bytes/sec" counters. Click OK when done.
You will have 4 new lines in the graph for the 4 counters you just added. Repeat the steps you did earlier and watch the graph to see if the Performance Monitor is acting the way you noticed. If it is, the problem is that this is the way Windows is reporting your disk activity and there is not much you can do about it. If it is different, the problem could be with the PerfMon plugin.
Statistics: Posted by SilverAzide — Yesterday, 11:42 pm