Can you apply the following patch (to the "next" branch of the
btrfs-progs git repo), rebuild, and try again? It's just adding some
debugging output to track what it's looking at.

   Hugo.

diff --git a/mkfs.c b/mkfs.c
index 2e99b95..51a5096 100644
--- a/mkfs.c
+++ b/mkfs.c

@@ -422,6 +422,7 @@ int main(int ac, char **av)
 	printf("WARNING! - see http://btrfs.wiki.kernel.org before using\n\n");
 
 	file = av[optind++];
+	printf("Checking whether %s is part of a mounted filesystem\n", file);
 	ret = check_mounted(file);
 	if (ret < 0) {
 		fprintf(stderr, "error checking %s mount status\n", file);

diff --git a/utils.c b/utils.c
index fd894f3..7fa3149 100644
--- a/utils.c
+++ b/utils.c

@@ -610,12 +610,16 @@ int resolve_loop_device(const char* loop_dev, char* loop_file, int max_len)
 	int ret_ioctl;
 	struct loop_info loopinfo;
 
+	printf("Resolving loop device %s (length %d)\n", loop_dev, max_len);
+
 	if ((loop_fd = open(loop_dev, O_RDONLY)) < 0)
 		return -errno;
 
 	ret_ioctl = ioctl(loop_fd, LOOP_GET_STATUS, &loopinfo);
 	close(loop_fd);
 
+	printf("Loop name = %s\n", loopinfo.lo_name);
+
 	if (ret_ioctl == 0)
 		strncpy(loop_file, loopinfo.lo_name, max_len);
 	else

@@ -639,6 +643,9 @@ int is_same_blk_file(const char* a, const char* b)
 		return -errno;
 	}
 
+	printf("Realpath of %s was %s\n", a, real_a);
+	printf("Realpath of %s was %s\n", b, real_b);
+
 	/* Identical path? */
 	if(strcmp(real_a, real_b) == 0)
 		return 1;

@@ -680,6 +687,9 @@ int is_same_loop_file(const char* a, const char* b)
 	const char* final_b;
 	int ret;
 
+	printf("is_same_loop_file: %s and %s\n", a, b);
+	printf("PATH_MAX = %d\n", PATH_MAX);
+
 	/* Resolve a if it is a loop device */
 	if((ret = is_loop_device(a)) < 0) {
 	   return ret;

@@ -784,8 +794,10 @@ int check_mounted(const char* file)
 			if(strcmp(mnt->mnt_type, "btrfs") != 0)
 				continue;
 
+			printf("Testing if btrfs device is in the dev list: %s\n", mnt->mnt_fsname);
 			ret = blk_file_in_dev_list(fs_devices_mnt, mnt->mnt_fsname);
 		} else {
+			printf("Testing if non-btrfs device is block or regular: %s\n", mnt->mnt_fsname);
 			/* ignore entries in the mount table that are not
 			   associated with a file*/
 			if((ret = is_existing_blk_or_reg_file(mnt->mnt_fsname)) < 0)

diff --git a/volumes.c b/volumes.c
index 7671855..2496fbd 100644
--- a/volumes.c
+++ b/volumes.c

@@ -130,6 +130,8 @@ static int device_list_add(const char *path,
 		device->fs_devices = fs_devices;
 	}
 
+	printf("Device added with name %s\n", device->name);
+
 	if (found_transid > fs_devices->latest_trans) {
 		fs_devices->latest_devid = devid;
 		fs_devices->latest_trans = found_transid;

@@ -223,6 +225,7 @@ int btrfs_scan_one_device(int fd, const char *path,
 		*total_devs = btrfs_super_num_devices(disk_super);
 	uuid_unparse(disk_super->fsid, uuidbuf);
 
+	printf("Adding device %s to list\n", path);
 	ret = device_list_add(path, disk_super, devid, fs_devices_ret);
 
 error_brelse: