The print area and margin handling by both the HP T120 printer driver (V4) as the HP Instant Printing application (v. 4.2.1) is horrible. The driver does not seem to be able to auto fit the print to the actual print area (printable area = paper size – margins) of a specified paper format such as roll (610mm) or A2+ sheet (458mmx610mm). Photoshop can do autofitting (the HP Instant Printing app does not) but that will most of the time result in prints that are half printed or miss at least some part of the actual image. This is illustrated in the picture below.
I think that the HP T120 messes up during printing when the picture is sized to a larger area than then HP T120's print area. As the HP T120 does not do borderless, the print area is always smaller than the sheet or roll media itself. I seem to be able to get consistent result when I by hand make sure that the picture print size is smaller than the HP T120's maximum print area. I will demonstrate this in the HP Instant Printing Application in the images below. What you see in the image below is that both the selected print media and the print size of the picture are set to Super C/A2 (or A2+ in most of Europa). The yellow dotted lines indicate that parts of the picture cannot be printed. Fine. However, what happens it that the print will come out as show above, printed half or at least missing parts of the picture.
If you manually adjust the picture print size under "Fit to" to match the maximum print area of the T120, the yellow dotted lines are gone and you should be good to go, right? Well, yes if you print on roll. Bu no, if you print on large cut sheet. In that case, you need to add some extra mm's to make sure the picture is completely in the T120's print area. That means more trimming than indicated by the yellow dotted lines. See the sample below, that printed correct. Notice that I went all the way down to 580mm, while Super C/A2 (A2+) paper is 610mm in height and the T120's margin is 17mm on the foot (5mm on the sides).
On other issue I have with both the driver and the HP Instant Printing app is that they do not seem to handle rotation well. The image below illustrates that. While the image is in portrait and the selected paper size is Super C/A2 and the "Fit to" custom paper size was set to 580mm by 345mm, hitting the rotation button actually reduces the ""fit to" size as opposed to just switching width and height. I now rotate my pictures in Photoshop before opening them in the HP Instant Printing application.
As final remark, this is what HP says on their website on the issue. I do not think that fixes it at all. It does show that the driver and the HP Instant Printing app read the margins set in the picture itself. Therefore, make sure that you export you picture from Photoshop with the right resolution and size (a size that should within in the print area of the T120!).
Having said all this, I you have the time to set things right you should be able to print on large cut sheet media and on 610mm (24inch) roll media fine.