There are many problems that may appear when a hard drive is approaching its capacity. While I don't have any Panasonic hard drive recorders (I have five Magnavox and two Philips HDD/DVD recorders) I've read enough AVS Forum discussions concerning the Panasonic HDD models (and, of course the Magnavox and Philips HDD models) that it is my practice to keep hard drive contents below 85% of capacity in order to avoid complications. (Some time back I allowed my Philips 3576 hard drive to get so full that it showed 17 minutes of open space at LP! Fortunately, there were no complications but I immediately started dumping unessential material and following that there was quite an editing session followed by high-speed dubbing sessions where the "keepers" were dubbed to DVDs. It's now my practice to keep at least 40 hours of free hard drive space on all my Magnavox and Philips HDD models.)
The Panasonic U61 error is specifically related to DVD Drive operation. While the U61 error may appear due to general operational situations, there may only be an indirect relation to hard drive condition or perhaps the U61 error is only coincidental to hard drive problems with your DMR-EH55. With such errors as U61, U88 and U99 there may be problems related to use of inferior media, an overdue DVD Drive lens and hub/spindle cleaning, laser assembly condition, problematic ribbon cable contacts or a problematic Digital PCB.
Yes, you need to free up hard drive space or perhaps reformat the hard or even replace the hard drive if there are too many bad sectors. See the detailed discussions among owners of Panasonic HDD equipped recorders at the AVS Forum.
There are also many detailed, ongoing AVS discussions of various general Panasonic service, repair/rebuilding procedures (many of these illustrated tutorials I've contributed under my DigaDo identity). Existing AVS service discussions are frequently revised to include additional, corrected or updated information.
Ongoing revision is not available at MyCE, a serious drawback when errors or misinformation become "set in stone."
If browsing at the AVS Forum be sure to set the display/search option back more than thirty days in order to find relevant Panasonic HDD or general service discussions.