I had Ubuntu 12.04 installed as dual-boot OS on top of Vista on my laptop. Hacked the grub settings to default to Vista (instead of the default Ubuntu -- pain) on startup, and all was OK for occasional Ubuntu use for past 6 months. Then last week I got a strange message about 'lack of disk space' (~50MB free) when installing pxyplot, even though there was still about 6GB free disk space when I checked later. Then today the Ubuntu wouldn't load at all, and checking the HDD partitions in Vista it looked like the 15GB Ubuntu partition was now three smaller partitions! So, I got rid of those partitions and expanded the Vista partition to use the reclaimed space. Now can't restart ('grub rescue' appears and doesn't 'rescue' anything), so I'll have to do a boot recovery using a Vista installation CD. (Not a particularly user-friendly failure mode of the dual-boot installation!)

I now have to decide to either a) try installing ubuntu on the HDD again, but don't want to stuff up my Vista ever again, as that is my most used OS, or b) install Ubuntu on a 16GB USB 3.0 stick. Apparently performance from USB won't be as good as from HDD, and running OS from USB stick does lots of r/w so the stick may fail after a few years! Perhaps installing Ubuntu on live USB and setup to then run in RAM would alleviate the performance/USB lifespan problems?

If I create a live-USB for Ubuntu OS, will it boot off that when I restart the laptop with it plugged in? Or will I have to change the laptop setting for boot-order whenever I want to boot Ubuntu instead of Vista (that would be even more painful than the grub default boot order putting Ubuntu ahead of the existing Vista OS!)

-- update:

I recovered my Vista setup using Iolo SystemMechanic Disaster Recovery Tool, and created a bootable USB of Ubuntu 13.10 on an 8GB USB3.0 pendrive, with 4GB of 'persistence' to allow saving of settings, install some packages etc. It worked OK for a couple of test boots, but once I changed the time and desktop wallpaper, the next Ubuntu reboot crashed and I then couldn't get it to boot successfully. So I decided to install Ubuntu 12.04 LTS as a dual-boot again, but this time instead of partitioning the HDD and installing from an ISO DVD I used the wubi.exe tool to install Ubuntu as a dual-boot. Worked very well, although one oddity was that, despite asking how big the make the partition (20GB), the installed Ubuntu appears to be happily installed somewhere within the Vista NTFS file system (no partition shows up in Windows disk manager, and in Ubuntu disk management tool the entire 133 GB of HDD is showing, with ~40GB free space). A nice feature of installing the dual-boot using wubi is that the laptop now uses Windows boot manager on startup, with Vista as the default OS and Ubuntu happily listed as second on the list. So far so good.